System for reducing noise in the transmission of electric signals



Aug. 12, 1952 H. H. sco'r-r 2,606,972

SYSTEM FOR REDUCING NOISE IN THE TRANSMISSION OF ELECTRIC SIGNALS Filed Jan. 25, 1946 8 Sheets-Sheet 1 35 g 6 0) 3 0: 3 R d {I a;

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c c c o o CONTROL F'LTER CIRCUIT c INVENTOR.

Hermqn Hqsmer Scott BY; g z

' A 7' TORNE Y Aug. 12, 1952 H. H. SCOTT 2,606,972

SYSTEM FOR REDUCING NOISE IN THE TRANSMISSION OF ELECTRIC SIGNALS Filed Jan. 23, 194 8 Sheets-Sheet 2 r2 7 v /7 cr/f f0, 3 v A Q 80 {/7 4 FILTER AMPLIFIER c 0 c (8/ a We v5 AMPLIFIER r r w 7 77 .o c O O FILTER CONTROL o C o CIRCUIT 6 INVENTOR.

Hermon Hosmer Scott BY 6 e .4 TTORNE Y Aug. 12, 1952 SCOTT 2,606,972

SYSTEM FOR REDUCING NOISE IN THE TRANSMISSION OF ELECTRIC SIGNALS Filed Jan. 23, 1946 8 Sheets-Sheet 5 r fo /7 f CONTROLLED c CIRCUIT g/ i 6 f; o 9

A52 o o /6 CONTROL CIRCUIT Q rfa 280 CONTROLLED CONTROL FH'TER cmcunr Fig. 5

x50 aa 7 CONTROLLED SOUR E ,4? C|RCU|T g 0 A? /J'6 21 24 f (I O CONTROL 7 cmcun INVENTOR.

Hermon Hosmer Scott A TTORNEY Aug. 12, 1952 H. H. scoTT 7 2,606,972

:- SYSTEM FQR REDUCING NOISE IN THE TRANSMISSION OF ELECTRIC SIGNALS Filed Jan. 25, 194 8 Sheets-Sheet 4 C r50 r8? 5/ J? a J T I I i KY I v v i I I U z? I I 4T )3 l I I I I L- L l CONTROL CIRCUIT INVENTOR.

Hermbn Hosmer Scott A T TOR/YE Y Aug. 12, 1952 H. H. SCOTT 2,606,972

SYSTEM FOR REDUCING NOISE IN THE TRANSMISSION OF ELECTRIC SIGNALS Filed Jan.- 25. 1946 a Sheets-Shet 5 v INVENTOR. Hermon Hqsmer Scofl Ar o/M'Y H. H. SCOTT Aug. 12, 1952 IGNALS 8 Sheets-Sheet 6 Filed Jan. 23, 1946 ADJUSTABLE B. P. FILTER a w I 3 I M M M. in. 4 ll? g I nlihwl; Flilp o .N f/ M a I INVENTORMC. Hermon Hosmer Scott By ATTORNEY Aug. 12, 1952 H. H. scoTT 2,606,972

SYSTEM FOR REDUCING NOISE IN THE TRANSMISSION OF ELECTRIC SIGNALS Filed Jan. 23, 1946 8 Sheets-Sheet 7 /-407 /4Z w /J0 f/f FILTER AMPLIFIER Q- L A L n 0 I W CONTROL CIRCUIT FILTER: AMPLIFIER f FILTER AMPLIFIER r I 42 I 23 CONTROL F'LTER CIRCUIT F/g. l0

INVENTOR.

Hermon Hosmer Scofl BY m ATTORNEY I Aug. 12, 1952 H. H. SCOTT 2,606,972 SYSTEM FOR REDUCING NOISE IN THE TRANSMISSION OF ELECTRIC SIGNALS Filed Jan 25, 1946 8 Sheets-Sheet s IN VEN TOR.

' ATTORNEY.

Patented Aug. 12, 1952 I SYSTEM FOR REDUCING NorsE. IN THE TRANSMISSION OF ELECTRIC SIGNALS Hermon H. Scott, Lincoln, Mass.,assigi1or, by

mesne assignments, to Myron T. Smith, Concord, Mass.-, trustee Application January 23, 1946, Serial No. 642,961

' 8 Claims. (01. 178-44) 7 present invention relates to electric methods and systems, and more particularly to meth- Ode of and systems for attenuating or suppressing background noise, interference, and other spurious disturbances during the transmission,

the' recording, or the reproduction of speech,

music, video, and other signals. The term reducing or its equivalent will hereinafter be employed generically to include either attenuating or completely suppressing.

Background noise and other extraneous disturbances, at one time or another, are attendant upon practically every form of transmission, recording, or reproducing system. They occur, for example, during the transmission and the reception of the sight and sound channels of television, the presentation of sound-motion pictures, the transmission and reception of signals by wire, telephone, cable, or radio, in relay circuits, and in the reproduction of all types of recording. They have heretofore been attenuated at the expense of seriously impairing the fidelity with which the desired signals have been transmitted.

'The customary practice has been to restrict the ;frequency range of the system so as to provide -What has been considered to be the best com- 'promise between the fidelity of the'transmission land the suppression of the noise. :misehas not, however, been satisfactory.

The compro- To take an illustration from phonographic-record reproduction, the interference has consisted yolume or high-intensity levels of the reproduced signal; at the low-intensity levels, the human ear does not seem to notice the absence of the high-frequency components of sound. As, moreover, the spurious noises are very effectively masked, so as to be nearly or quite imperceptible to the human car, when the speech or music reproduced by the phonograph record is loud, it was proposed to attenuate the high-frequency components at the low-volume levels only; and to permit reproduction of the high-frequency range atthe high levels, either of the high-fre quency components themselves, or of thereproduced signals as a whole. l y This modification of the originally proposed compromise, however, can be practically availed of only ,when the desired high-frequency-signal components are ata high level comparedto the noise components, or are accompanied by mee dium-frequency components of substantial volume or intensity. What is meant by mediumfrequency components in this specification will be explained hereinafter. Whenthese loud me;- dium-frequency components are present, as be fore stated, the eardoes notnoticethe high-trey quencygnoise components. Thepro'posed modi-fl fication cannnot be availed Of generallyftherea fore, when the responseto the high frequencies is unaccompanied by a signal at medium 1 5? quencies of substantial level. g these previously proposed systems, high; level high-frequency interference signals have been reproduced without any attenuation thereof, because their very presence allows the system to reproduce the high-frequency ranges. Systems depending for their effectiveness upon control-in accordance with the level in the high-frequency range are fundamentally defective in that the greater portion of the control may be exercised by the noise-frequency components themselves, if the noise level is high enough. This permits of reproduction in the high-frequency range with accompanying spurious noise, when the level of the desired signal is low; and particularly so when there is'no desiredsignal at all tobe repro duced in that high-frequency range otherthan the undesired noise. During such periods of silence of the desired signal, even between notes in musical phrases, the high-frequency scratch is very annoyingly heard," and it is all thatis' then heard. It is very desirable to attenuate the very audible needle scratch that may accompany that music silence. The same considerations apply, however, even in periods of non-silence, where the volume level of the reproduced music contains no loud medium frequency components, and ,wherr'most of'the high-frequency musical components exist onlyat levels too low to be heard.

In copending applications, Serial Nos. 641,673 and 642,411, filed January 17, 1946, and January 19, 1946, respectively, the problem of eliminating these spurious noises is solved by a novel method of attenuating the high-frequency components of the signals. Each of. the said applicationsldis closed both variable filter and variablefamplify other interference during transmissi n,-record-- ing, reproduction, and the like, that shall retain practically a high degree of apparent fidelity.

To the attainment of this end, a feature of the invention resides in reducing the response in the high-frequency region, not in accordance with the level of the higher frequencies, and not in' accordance with the level of all the frequencies as a whole, as has heretofore been proposed, but rather in accordance with the level of some or all of, the;medium, orsome or all of the low frequencies, or the frequencies below therange of the high frequencies that are being controlled, or both types of frequencies. The noise'is suppressed when the level of these medium or'low frequencies is so lowthat the signal cannot mask the noise; or so low as to indicate that any probable level of the higher frequencies, insofar as the desired signal is concerned, is below the threshold of hearing. The noiseis thus reduced at low-volume levels by reducing the range of the high-frequency overtones that are not normally heard by the ear except at the high-volume levels." Better suppression of 'noise, with less effect upon the reproduction of thehigher treble tones, and with no apparent decrease in the quality of reproduction, is thus attained. If the signal is such that the level of the low-frequency components is high compared to that of the highfrequency components and, particularly, the noise components, the described operation, of course, takes place when the level of the signal as a whole falls below a predetermined level. I

Though theexample above has been given, for illustrative purposes, I in connection with the spurious noises attendant upon the high-frequenoy components of the signals coming from a phonograph record, the invention is notlimited to such use. The invention is of general applicability. It may be used to advantage to. reduce the noise attendant uponthe reproduction of practically any type of system involving recordings made upon wax, discs, tapes, films, wires, etc.- It maybe used also to reduce various types of interference common in radio-broadcasting, such as adj acent-cha'nnel interferencegfrequently called monkey-chatter," heterodyne whistles, the effects of atmospheric and Y other. disturbances, and in any other application where 'noise isafactor. It is equally useful at audio, video, and other frequency ranges. H I L The low and the medium frequencies are not suppressed or alteredin the process of eliminating the high-frequency componentsof thesignals at the low-volume levels.. Though thus effectivelyeliminating the spurious noise,. 'therefore, the elimination of the high frequency components may, under some conditions, if au'dible components are in any way affected-result in loss of apparent brilliance of the reproduced signal, and in a loss of balance or symmetry between the high-frequency and the low-frequency responses. w

, This may bra-"understood by considering, for example, audio-frequency music signals, the'relative balance of which is referred to as aural bal-- ance. Aural balance may be defined as the ratio of the audible energy in the treble notes to the audible energy in the base notes; or the ratio of the reproduced high and low frequencies. Those components only of the energy are taken into account, in this ratio, as are of such frequency and amplitude as to correspond to audible tones at the particular levels at which they exist.

Another object of the present invention is to provide a new and improved method of and system for reducing or suppressing background noise cr -other interference of this character accompanying the low-frequency components of the signals. 1 These may, for example, in music, include mainly the range of bass fundamentals, below about-250 cycles, say, in the region where the human ear is relatively insensitive at low levels.

Since such low frequencies are seldom present in music except when accompanied by harmonics residing in the medium-frequency or the higherfrequency regions, this control may be effected,

in accordance with the level of the medium or the high frequencies.

It has been explained that systems eliminating high-frequency noise, through control by high. frequencies, are unsatisfactory. For the same.

reasons, control of the low-frequencyv responses... to eliminate undesirable low-frequency.noise,;by; Such the low frequencies is unsatisfactory. would result in the reproduction of undesired loud low-frequency noises, even though node-1 sired signal may be present. The elimination of the low-frequency noises may similarly resultin destruction of the aural balance, this time result: ing from the attenuation near the low-frequency limit of the useful frequency range. I

Any modification of the reproduction of either; the treble or-the base notes, for a particular level. of reproduction, may produce an obvious change, in the aural-balance-ratio relationship of the reproduced high and low frequencies. This may become noticeable to the car as an apparent, change in balance, tending to make the reproduction either high-pitched or low-pitched, resulting either in an unduly brilliant or an un-. naturally dull or boomy quality, depending upon whether the attenuation is more'efiectiveat. the low or the high frequencies. This is par-v ticularly annoying to music-lovers, and tends to obscure fine details in musicr In'prior-art systems, the operation of the noise-.- suppressing means has been accompanied b'y a shift of the balance between the high and the. low frequencies that is often very noticeablefito the ear and renders thepresence 'ofthe noises. suppressing system apparent even when its operation is otherwise satisfactory. 1

If the same listener always heard the program over the same range of levels, and the orchestra always produced exactly the same spectrum, it would be a simple matter to'de'termine whatfrequenoy ranges could be attenuated or suppressed, without afiecting the qualitynoticeably. In' any practical application, however, many v'ariations prevail. The usual type of signal, for -i'nstance, does notremain constant. All listeners, again, do'not'have equal acuity of hearing. The-total sensitivity of the system, from microphone to loudspeaker, moreover, may be varied, or the volume range of the program may be contracted, thus changing the range of listening levels. These factors may result in a condition where sometimes, the transmitted frequency range ot-the music is less than the-range of the components that would be. perceived by the ear. This would impart an unnaturally low-pitched or high- Another object of the present invention-there fore, is to provide a novel method of and system for maintaining a substantially constant balanced relationship between the volume levels .of the high and the low frequencies.

Another object of the invention is to provide a novel method of and system for maintaining a substantially constant apparent frequency range in reproduced music and other applications, though actually restricting the range under conditions where noise would be troublesome.

This is especially desirable in practical systems involving the reproduction of music where a change in the balance may be objectionable, particularly if the modificationseems to vary with the volume level of the sound. The aural balance between the energy in the treble notes and the energy in the bass notes obtained in accordance with the present invention yields a high degree of apparent fidelity such as to satisfy the ear, even under conditions where frequencies within the audible range for the particular level to be reproduced are attenuated to a degree that would otherwise be noticeable. This allows greater suppression of noise with less noticeable effect upon musical quality than in prior-art systems.

To the attainment of the above ends, a further and important feature of the invention'resides in the simultaneous control of both the high and the low frequencies or, alternatively, the introduction of a predetermined amount-of resonance into the response of the system when the range is restricted, or a combination of both methods. Although the frequency range may be restricted,

under some conditions, relative to other condiflexibility of the system with regard to the type of signal, listening conditions, etc. l

The desired ends may-be attained, in accord ance with another feature of the invention, by"

suitably varying the characteristics of suitable amplifying means.

In accordance with a feature of this invention, better suppression of noise is obtained, with no apparent decrease in the quality of reproduction, not only insofar as the high frequencies, but also insofar as the low frequencies, are concerned.

Other and further objects will beexplained hereinafter, and will be more particularly pointed out in the appended claims.

The invention will now be more fully explained inconnection with the accompanying drawings, in which Fig. 1 presents a series of graphs representing the frequency-response characteristics of the human ear, and the relative amplitudes and frequencies of the various component presentin a typical signal, such as orchestral music, the abscissae being plotted logarithmically in terms of cyclesr-per-second frequency, and the ordinates being plotted arithmetically, in terms of decibel intensity units; Fig. 2 is a. block-diagram viewof a typical system embodying the present invention and representative of the circuits and apparatus of Figs. ,7 and 8,; Fig. 3 is a further modification of Fig. 2; Fig. 4 is a view representatlve'of an alternate method of connecting the control circuit in the systems of all of the figures except.- Fig. 1; Fig. 5 is a view representative of all the.

other figures except Fig.1; Fig. .6 is a similar view of a modification; Fig. 7 is a diagrammatic view of circuits and apparatus embodying the form of the invention illustrated in Fig. 2'and showing actual typical circuit elements; Fig. 8 is a similar view of a modification; Fig. 9 is a similar view of a form of the invention combining a variable amplifier and a variable filter; Fig. 10

is a view similar to Fig. 3 of a further modificae tion; and Fig. 11 is a view showing a typical cir-' cuit configuration similar to Fig. 3.

Though the method and system described here in areapplicable wherever the degree of perceptionof the signal-receiving means varies With-the characteristics of the signal, and wherein balance between the high andthe low frequencies, or

two bands of frequencies, is'important; the inventicn, for concreteness, will be explained as it ap- From the curve I of Fig. 1, representing the threshold of hearing-sensitivity for an ordinary human ear, it appears that the ear is most sensitive in the region around 2,000 cycles, and that" the sensitivity falls off rapidly at both lower and higher frequencies.

The curve 2 may be considered to be representative of the energy-distribution characteristics, as a function of frequency, of typical orchestral music at a particular. level. When shifted along the axis of ordinates, to correspond to changes in the volume or intensity of the sound, as in re-,

sponse to a volume control or an attenuator, it

intersects the threshold curve i at points representing different frequences. These points represent the limits of the frequency range necessary to reproduce satisfactorily for thelistener the particular type of signal at the particular volume or intensity levels. If the level. ofthe curve 2 is reduced equally throughout the range, for instance, to occupy the position of the curve 3, the necessary or perceptible range of frequencies isseen to become accordingly restrictedfrom about 70 cycles to about 250 cycles at the low-frequency portions of the curves 2 and 3, respectively, and from 12,000 cycles to, about 4,000 cycles at the high-frequency portions of the curve 3. Ifthe music or other signal should have frequency components outside these ranges, they would notice audible.

-Most musical instruments, when played softly. moreover, produce a mellower tone than when played loudly, thus indicating the presence of fewer harmonics or overtones falling within the higher-frequency ranges. While the curve 2, as before stated, may become transformed into the curve 3 when the sensitivity of the transmission means is reduced, therefore, it may actually become transformed into the still lower curve 4 if of Fig. 7, the filter 201 associated with the amplifier-40 is low-pass, and the filter 301 associated .with the amplifier M is high-pass. In the system of Fig. 8, the filter 201 is band-pass and the filter 301 is band-elimination.

Inthe system of Fig. 7, the filter 201 associated with the amplifier 40 is shown low-pass, and the filter 391 associated with the amplifier M is shown high-pass, for the control of'high frequencies only. This filter 202 is connected; in any desired manner, to theinput circuit ofthe vacuum tube 19 of the amplifier 40. The filter 201 of Fig. 7 typically comprises a series inductor 18 shunted by a condenser 38, and a shunt condenser 45, providing an m-derived half sectiongiving sharp cut-off at, say, 2,500 cycles and high attenuation above cut-off. By proper choice of the operating impedances for the filter 201 a predetermined amount of resonance may be introduced into the circuit to increase the amplitude of transmission or lengthen the duration of transients at frequencies near the cut-off frequency, thus to improve aural balance. The filter 301 is of a simpler type, comprising a' series condenser 46 and a shunt resistor 41. The filter 301 transmits mainly frequencies above the cutoff of the filter 201. In the system of Fig. 8, however, the filter 201 is shown comprising a condenser I18 in series with the inductor 18, and an inductor I45-in parallel with the capacitor 45, and the filter 301 is shown provided with a series arm comprising'a parallel-connectedinductor I 43 and capacitor 46 and a shunt arm comprising a series-connected inductor 41 and capacitor I41. In the system of Fig. 9, the filter 301 is shown constituted merely of 'a series 1'6- sistor 25I and a shunt capacitor 252.

The series arm of the filter 201 is shown connected to the control grid 98 of the vacuum tube 19 of the amplifier 40. A resistor 43, connected between the anode 90 and the cathodeIIJI of the vacuum tube 19, is connected across the output conductors B and BI. The series arm of the filter 301 is shownconnected to the control grid 205 of the vacuum tube 203 of the amplifier 4|. A blocking condenser 268 may be interposed.

A resistor 209, connected between the anode 204 and the cathode 206 of the vacuum tube 204, is connected across the output'conductors I3 and 14.

The attenuation or suppression is under the control of a control circuit 6. The control circuit 6 is of such nature as to produce no noticeable effect upon the quality of the reproduction. The input side of the control circuit 6 is subjected to the action of the input signal from the signal source I9 through the medium of lead conductors 52 and 53'respectively connected to the input leads 50 and 5|. The control circuit 5 is designed so'as automatically to shift the response range of the controlled circuit 10.

The control circuit 6 is shown in Fig. '1 as comprising a series rectifier 89 for rectifying the signal, connected in the control-circuit-input lead 52, and acondenser I4 connected across the control-circuit-input leads 52 and 53. The condenser M, which bypasses signal voltages between conductors 23 and 24, may be located in associated circuits. The resistance of the rectifier 9 and the capacitance of the condenser I l provide an R-C filter circuit for reducing the high-frequency components of the rectified signal voltage. A leak resistor I3 is connected in parallel with the capacitance Id. The rectified voltage of the control circuit 6 varies'in accordance with 10 the amplitude of the signal applied to the input terminals I5 and I6. Additional filtering may be added in the lead conductors 23 and 24, connecting the control circuit 6 to the variable amplifier 4| or additional amplification in the leads:

52 and 53, if desired.

In the systems of Figs. 8 and 9 the rectifier 89 is shown connected in parallel with the parallelconnected condenser lyl and resistor I3. A further series arm comprising a resistor BI and shunt arm comprising a capacitor 92 is also shown provided in the systenrof Fig.-9. -A- further such control circuit 405 is shownjprovided tem'should contract at liloximately the same rate as the decay of reverberationin normal music. Since the control voltage isobtainedby rectifying the specified frequency components it is obviously a variable D. C.-:bias varying in accordance with the level of said components. t

The output of the control circuit 6 is shown connected, in Fig. 7, byv the conductor24 to the cathode 2830f the vacuum tube 203 of the variable, amplifier II and,- "by the conductor 23, to. a further control grid I63. In-the' systems of Figs. Sand 9, however, the connection of the conductor 23 is shown to the control grid 205, through a resistor I2. The grid bias thus 'developed in the system of the control circuit 6 is therefore trans-- mitted by theleads23 and 24 to: the variable am- "plifier 4| to impress and adjustable controlvoltage from the output of the control circuit 6 upon the grid'I03.

The adjustments are such that the control circuit 6 automatically adjusts the system sofas .to attenuate all frequency componentsof J the the signal.

input signal above al'predetermined value, such as 2,500 cycles, at low-volume or low-intensity levels of the signal, orpredetermined rangesof The spurious noises accompanying 4 the high-frequency components are thus eliminated at low-volume levels. .withalminimum effect upon the tone quality. The system thus adjusts itself automatically to the signal-to'be transmitted.

The characteristics of the control circuit 6 should be so selected for each range as to insure the best performance with the intended'type f signal. i: -60

A filter network 1 may be inserted in the lead conductors '52 and 53, in the input circuit of the control circuit 6, tovary the response of the'controlled circuit 6 with frequency. The filter1-may comprise aseries resistor 2| (Fig. '1) or inductor I2I (Fig. 8) and a shunt capacitor 22, and' also. as shown in Fig; 9, additionalleleinents. The functionof the-filter 1 is to prevent the-control circuit 6 from operating on signals in the-eliminated range of frequencies and'may approximate the sensitivity characteristics" ofthe can: This "is necessary when the applied signal may have strong high frequency or l'o-w-frequency"noise components occurring at levels higherthan the desired components. An amplifier 2 IIID- may also be inserted in these leads 52 and*53 to'increase 1 1 the magnitude or theefiectiveness of the control voltage. A filter (not shown) may also be added in theoutput leads 36- and 56 of the control cir- 'cuit shown in Fig. 6, if desired.

The control circuit 6, which controls the transmission of the amplifier AL-is thus actuated by signals obtained through filter 1. The purpose of this andsimilar filters is to reduce the sensitivity of the control means in the frequency ranges being controlled, thus to prevent the transmission of high-level noise in those ranges.

As' illustrated in Fig. 6, however, a result similarto that obtained with the aid of the filter 1 may be obtained without employing the filter 1. The leads 52 and 53 are shown in Fig. 6 connected to the output terminals I1 and I8, as indica'te'd by the leads 36 and 56. The control voltage is then derived, not from the input, but from the; output'voltage of the system, under the control of the output signal, at the output level. It may also be derived from a voltage at any point intermediate between the input and the output terminals, depending upon the type of control "characteristics desired, as will later be described "in connection with Fig. 9. The terminals I and I0 may represent the input and the terminals I1 and [8 the output, or vice versa.

When the control is from the output circuit, 'thesystemcan expand in frequency range only in thepresence of strong signals in the range not controlled. It may contract, however, only when all of the signal components decrease'in amplitude. While this operation is not identical with control by the input circuit, through the filter I, it i's similar inthat high-level'noise in the controlled range cannotcause the system to expand. If', hcwevena filter I is added to the leads 3.6 and 5.8-, the operationwill be similar to that of'the =systenrof.IEig. 5.

' The control of the control circuit '6, actuated, *as'before described, in accordance with the amplitude of the applied signal, is such. as to permit the restricted-range amplifier 40 tov operate only at low levels, and both amplifiers to operate at high levels. The overall response of the system will thus vary with the level of the applied signal. Those frequencies only will tend to be included thatare necessary for satisfactory reproduction of thesignal at the'volume level existing at any particular moment. At'lowvolume levels, therefore, the restricted-range amplifier 40 is alone iii-"operation. The. high-frequency components,

with their accompanying noises, are eliminated by r'easonof the inoperability of the amplifier 4 at this time, to transmit them.

The low frequencies maybe similarly controlled. When both the high and the low fre- "quencies are controlled, or when the filters associated with the low-level amplifiers have resonant cut-off characteristics, substantially con- :5

the use of band-pass and band-elimination filters for controlling both the high and the low frequencies, while maintaining balance. The range of thefixed amplifier All-may be restricted by the band-pass filter fill -comprising, as before explained, a series inductor 18 connected inxseries with a condenser I18, and a shunt capacitor 05 connected in parallel with an inductor 45. Mediumfrequencies may be eliminated from the variable-gain amplifier 4| by the band-elimination filter 301, comprising, in this case, as already explained, aseries capacitor 46, connected in parallel with an inductor I45, and ashunt inductor 41, connected in series with a condenser I41. This filter 301 may be connected in any desired manner, to the input circuit of the vacuum tube 203 of the amplifier 4|.

The controlled circuit 10, therefore, may be designed to pass normally only a, restricted range of frequencies at low-volume levels. This may be expanded to the full range that it is desirable to transmit at high-volume levels; say, from about '10 to about 12,000 cycles, or from about to about 8,000 or more cycles, or any other range, depending upon the application and the limitations of associated equipment. The controlled circuit 10 may also be designed so that it shall be able to attenuate or completely suppress, at low-volume levels, in predetermined ratio, the higher and lower rangesof frequencies; say, the high-frequency components above about 2,500 cycles, and the low-frequency components below about 250 cycles, or some similar values, depending upon the application and other factors.

The fixed-type amplifier 10, therefore, transmits, as a unit, one or two of the three types of frequency components heretofore described as high, medium, and low. The variable-type amplifier 4| transmits the other one or two types as a unit. The control is such that that unit is reduce dthat does not contain the medium-frequency components. Thi is effected when the level of the medium-frequency components or the low-frequency components or both falls below the predetermined volume or intensity level; or when the level of the signals falls below the predetermined value if the signals are of such character that the level of the low-frequency components is high compared to the level of the high-frequency components.

Preferably, as in the system of. Fig. 8, the fixedgain amplifier 40 will transmit, the medium-frequency components only, and the variable-type amplifier will transmit, as a unit, both the highfrequency and the low-frequency components. This effects control of both the highand the low-frequency noises, and also allows maintenance of the balance between the high and the low frequencies under all conditions.

These control circuits are interchangeable and may be used in the systems of any of the figures, with or without extra amplifiers orfilters.

According to the system of Fig. 3, both amplifiers 15 and 42 are variable. The fixed amplifier 40 of Figs. 2, 7, and 8 is replaced by the variablegain amplifier 15, having a narrow range, representing only the minimum range required for lowvolume-level reproduction. It may provide for increase in amplification in a portion of this range, as by means of a resonant filter, thus intensifying certain frequency ranges when others are attenuated. As previously mentioned in connection with Fig. 7, the filter 201 may have resonant characteristics accentuating a band of frequencies in the neighborhood of cut-01f. For this particular filter such resonance can be obtained when the impedance across leads 50 and 5| is relatively low, or the output impedance across capacitor 45 is relatively low with respect to the normal terminating impedance for a filter of this type. This would improve balance. The variable-gain amplifier 4| is replaced by the variablegain amplifier 42, though without the filter 301. The amplifier II has a wide range, operating at high-volume levels, for transmitting the entire range of frequencies that are desirable for highlevel reproduction. The control circuit 6 is connected to the variable amplifier 42 in the same way as to the variable amplifier 4| of Figs. 2 and 7. The control circuit 6 is connected also, however, to the variable-gain amplifier 15 by corresponding separate control-output conductors .16 and 11. j

The filter I (if employed) and the control circuit 6 turn on the wide-range amplifier 42 in the same manner that they turn on the amplifier 4| of Fig. '7. They turn off the narrow-range amplifier 15 through the control-output-connecting conductors l6 and TI, when the signal contains strong frequency components within the range of the narrow-range amplifier 15. At low-volume or low-intensity levels, therefore; the restricted range of the amplifier I5 will reduce or attenuate the high-frequency components. At the high levels, the amplifier I5 will become substantially inefiective, and the wide-range amplifier 42 will then alone function. The .combined frequency characteristics of the two amplifiers I5 and 42, under these conditions, should be such as to provide the desired overall response at any particular intermediate level. In the system of Fig. 3, therefore, the control circuit 6 renders one of the amplifiers l5 and 42 substantially ineffective when the other is effective, and vice versa.

The filter I insures that the control shall be in accordance with signals in the narrow range. In the system of Fig. 3, too, under certain circumstances, the filter 1 may be eliminated, or the control circuit 6 may be actuated from the output of the narrow-range amplifier l5.

The main advantage of the system of Fig. 3 over that of Fig. 2 is that the narrow range may contain a certain amount of resonance, thus improving the reproduction in the neighborhood of the cut-off frequency at low levels. Such resonance, however, is undesirable at high levels, under which conditions the amplifier v"l5 is essentially turned ofi. v

Fig. 11 illustrates typical circuit configurations applicable to Fig. 3. The filter 201, comprising inductors I8 and I45 and capacitors I18 and 45, is a band pass filter transmitting only the narrow range required at low levels to amplifier I5. The constants of this filter may be so chosen as to provide a certain degree of resonance in the region of cutofi. Similarly, other circuit configurations, including resistance-capacitance and resistanceinductance filters may be used.

The amplifier 42, on the other hand, transmits a relatively wide range. Both amplifiers are under control of the control circuit 6 so that when amplifier T5 is transmittingthe full level, amplifier 42 is substantially cut off and vice-versa. Under intermediate conditions, both amplifiers provide partial transmission. For the sake of variety, filter I has been eliminated and control derived directly from the output of filter 201 through leads 84 and 85 as shown in Fig. 10. Filter 1 may, however, be used and may typically comprise a configuration such as filter 207 or such as the filter I in Fig. 9. Preferably, filter I should restrict the control to essentially the range transmitted through filter 201. l

The control circuit 6 is similar to the combined control circuits 6 and I06 of Fig. 9 except that the rectifiers 89 and I89 are seriesrather than parallel-connected for the sake of variety. Rectifier v39 r duces a negative voltage across condenser *14 I4 and resistor I3 in' accordance with the signal applied through leads. and 85. This voltage is filtered by'resistor 9| and condenser 92 and applied between the grid 305 and the cathode 306 of vacuum tube 303 in amplifier 15 through leads J6 and 11. Similarly the other htur of the control circuit, with numerals augmented by I00, applies a positive bias in accordance with the signal to the grid 405 of tube 403 through leads 23 and 24. Thus one amplifier is turned on when I the other is turned off and vice-versa.

Alternative arrangements or adjustment of the circuit values of filter 20! will allow filter 20l to function'substantially as a low-pass or a highpass filter when only one end of the frequency range, reversed in phase, so as to reduce the overall range of the system. A variablet tube or other variable amplifier, moreover, may be used in the feed-back circuits of amplifiers {not shown) using feed-back,,thus controlling the gain by controlling the feed-back.l

The system of Fig. 10xembodies the fixed amplifier 4D and the variable amplifier 4|. of Fig. 2, with their respectively associated band-pass and high-pass filters 2H? and 30?, under the control of the control circuit 6, with its asosciated filter I, though the control'exercised by the control circuit 6 is of a modified rating. A further variable-gain amplifier I 42, however, is connected in parallel with the amplifiers, 4B and M, in series with an associated filter'lll, and controlled by a further control circuit 88. To this end, the control circuit '86 is shown connected to the amplifier I42 by conductors 6'! and 83, and to a" point between the amplifier 49 and the filter 201 by conductors 84 and 85 as a further variant on the connections of Figs l, 5, and 6.

Three transmission paths or channels are thus provided, several with separate control circuits 6 and 86., The low, medium, and the high frequencies are reproduced through these separate channels. The amplifiers I42 and M are substantially ineifective at low levels. The channel provided by the amplifier as is substantially that required for low-volume-level reproduction. The two channels provided by the amplifiers 4! and I42, when operating, extend the range to that required for satisfactory high-volume-level reproduction. The amplifier I42 and its associated low-pass filter 4:11 may transmit the extra low frequencies required for high-level reproduction and the amplifier 4| the extra high frequencies, or vice versa. As the low-frequency and the high-frequency channels are ineffective at low-volume levels, such as are represented by the curve 4 of Fig. l, therange is restricted to that of the medium-frequency channel '40, thus reducing transmission at frequencies above and below predetermined values;

Two methods of control are provided in the system of Fig; 10. There is first the control represented by the system of Fig. 5, resulting from the location of the filter'l in association with the control circuit 6. Secondly, the control circuit 86 provides operation somewhat'similar to that of the system ofFig. v6, but with control from a point intermediate between the ,ifilter 201 and the amplifier 40, rather than from the-output, as'shown in Fig. 6.

The control circuit 86 is controlled from the output of the filter 201, in the fixed channel.

Control is thus obtained from a point between .the input. and the outputcircuits of this channel, thus eliminating any need for a special filter like I. e

In practice, either the amplifier I42 or the amplifier 4i may transmit the high frequencies and the other the low. Either amplifier may be controlled from the input, the output, or some intermediate point in this system. So long as the; reproduction of the highv and low frequencies is maintained ina predetermined relationship,

aural balance may be maintained as described in the saidv application, Serial No. 642,411.

Though optimum operating conditions are provided by the system of Fig. 10, 'the adjustable I paths or channels could, of course,'be controlled from a single control circuit, as in the systems of Figs. 7 and 8.

The system of Fig. 10 has the advantage over the systems shown in Figs. 2, 3, and 8 in that the high-frequency channel maybe controlled more rapidly than the. low-frequency channel,

thus to improve the reproduction of high-frequency transients.

The variable amplifiers may be combined with the variable filters described in the said applications, Serial Nos. 641,673, 642,411, and 642,412.

According to the system shown in Fig. 9, the fixed filter 201 and the amplifier 40 of Figs. 2, '7,

and 8 are replaced by a variable band-pass filter I69, under the control of a separate control circuit I06. The variable band-pass filter I69 comprises a seriesarm containing an inductor I0, connected in the input lead 50, and tuned by means of a parallel-connected capacitor 38 to provide a parallel-resonant circuit. This provides a corresponding fixed point of high or substantially infinite attenuation above the cut-off of the normal transmission range, where the capacitor 38 resonates with the inductor ID. The band-pass filter comprises also a separate arm, connected across the output leads .80 and BI, and

[comprising a second inductor 34, connected in series with a conductor 25, leading from the output conductor 80, and therefore in series with the variable reactance of a reactance tube 8, provided with a feed-back network comprising a condenser II between its anode or plate 54 and its control-grid electrode 55. The anode 54 and the condenser I I are connected in parallel to the output lead 80 by the conductor 85, through a capacitor 35 and the inductor 34, and the cathode 51 of the tube 8 is connected to the input lead 5| and the output lead ill by a conductor 58.

fore, functions as a double-m-derived filter with a sharp high-frequency cut-off characteristic.

The fixed point of high attenuation should generally be placed above the normal-operating range. For reproducing ordinary shellac phonograph records, which contain frequencies up to approximately 6,000 cycles, 9,000 cycles is a good value for this purpose. If the system is to be used also for the reception of amplitude-modulated radio broadcasts, 10,000 cycles, which is the difference between adjacent channels, is a desirable value, as-the 10,000-oyc1e attenuation will reduce also the heterodyne whistles. For program sources involving higher frequencies, of course, this fixed attenuation point may be set at a still higher frequency. I

The variable filter I59 passes the medium-frequency range and as much of the lower frequencies as is required for low-level reproduction. At high levels, the filter I69 passes also the, high frequencies, and the variable amplifier M also transmits, thus extending the low-frequency range. The filter 301, in this case of the simple R-C type, connected with the amplifier 4|, restricts its range to the low frequencies.

The control circuit 6 is shown controlling the filter I69 and the control circuit I06 the amplifier M. The conductor 23 is connected to the control electrode 55 through the resistor I2, and the condoctor 24 is connected to the cathode 51.

The adjustments are such that the control circuit 6 automatically adjusts the cut-off frequency of the filter I69 so as to attenuate all frequency components of the input signal above a predetermined value, such as 2,500 cycles, at low-volume or low-intensity levels of the signal, or predetermined ranges of the signal. The spurious noises accompanying the high-frequency components are thus eliminated at low-volume levels with a minimum effect upon the tone quality. The system thus adjusts itself automatically to the signal to be transmitted.

It is generally desirable to obtain the system shift from narrow-range to wide-range reproduction at a high rate of speed. This is a particular advantage of the type of variable filter I depicted, as has been more fully explained in the said application, Serial No. 641,673. A predetermined degree of resonance may also be introduced into the filter, as disclosed in the said application, toprovide improved aural balance. Both of these characteristics may be improved by operating the filter from a low-source impedance, or by keeping the impedance I03 at a relatively low value.

The filter I shown in Fig. 9 may be made, in connection with the control circuits, to approximate the characteristics of the ear, the low frequencies being attenuated by a series capacitor I91 and the high frequencies by the shunt capacitor 22. A shunt resistor I2 I is connected between the capacitor I91 and the resistor 2I. The various filters I may be used interchangeably in the various figures, or equivalent circuits may be used. The amplifier I00 may be used to increase the sensitivity of the control.

Essentially similar results, therefore, so far as steady-state conditions at low and high levels are concerned, may be obtained with either variable filters or variable amplifiers.

There is one important distinction between the operation of variable filters, as characterized by the system of Fig. 9, and the operation of variable amplifiers or variable filters in which resistive elements are varied as described in said copending applications. In a filter such as is illustrated in Fig. 9, either with or without points of high attenuation, the variation takes the form of a variation of cut-out frequency, maintaining reasonably constant transmission up to the cut-off .be; utilized, unbalanced or push-pull. A" filter ma be employedto' control one frequencies, for'i'nstance, anda variable mph- .frequency, and rapidly, increasing. attenuation above thatifrequencya With variableamplifiers,

or with filters employing. variable ,r esist anc.e,=' the cuteofi frequency remains substantiallyafixed,;and the changeEin the all-over transmission characteristics is accompanied merely by varying the. at-

' tenuation above the. cut-oif frequency. .While tions; In .some cases, a'cjoi'nbination oihthe' jtivo type's' may be justified. Various combinations'iof fixed and variable filters and" amplifiers .also may gable fie'r to control another range of frequencies; or yic'e'Versa.

' "'It' is' also understood thatftubes with v'arioius control electrodes may beutiliz'ed and that the signal and control voltages may be applied to the same or separate electrodes, or that other well known means such as current-controlled resistors or impedances may be used to vary the transmission of the various channels.

In accordance with usual conventions in the showing of vacuum-tube circuits, the well-known operating-voltage sources, like batteries, power supplies, voltage dividers, bias resistors, transformers, and the like, are not illustrated. It will also be understood that, in any practical system, it may be desirable to add amplification or attenuation, so that the best operating conditions may be obtained.

Further modifications will also occur to persons skilled in the art, and all such are considered to fall within the spirit and scope of the present invention, as defined in the appended claims.

What is claimed is:

1. An electric system for transmitting signals having three types of frequency components, namely, relatively-high-frequency components, relatively-low frequency components, and components of medium frequency, the system having, in combination, means for transmitting two of the said types, means for transmitting the third .An. e-1ec rici;sy.s..t.em:rfon transmitting isign type, one of the transmitting means for transvice to be transmitted by said electron-discharge device, and means controlled by said mediumfrequency components to vary said control bias applied to a control electrode in said electrondischarge device to reduce the transmission in said electron-discharge device and consequently said other transmitting means when the level of said medium-frequency components falls below a predetermined value.

2. An electric system for transmitting signals having. I three. .types :of, frequency components, namely, relatively high-frequency components, relatively. low-frequency components, and. com- .ponentsxofzmedium frequency;.the;.system havin inacombination, means for. transmitting two of the. said: types, means, for transmitting the third type, one of the. transmitting means transmitting the medium-frequency component substantially unchanged, the other transmitting means comprising an-electron dischargedevice transmitting said others-type icomponents. :controllable. by; a variable control bias thereby; controlling said itransmission of said other-type. 1 components, means for applying the signals to be transmitted by said.- other;v transmitting .meansto a control electrode. in. :said electron-discharge, device to ,be transmittedby said. electronedischargedevice, and

means; cont-rolled by the signals in the transmit ting means transmitting the. medium-frequency .jcomponents-to ,varycthe control .bias .applied- .topa control electrode in: said .1 electron-discharge Adevice to .reduce :the transmission insaidelectronedischarge zdevice .and consequently ,said,..;other transmitting means when ,saidcontrolling signals fall.belowapredetermined value. vI- having; three. tvpesgpf'...ir qiiency. mp s, namelm: -.relativelye eh-i e uency. compo e ts. relatively-low-frequency components, and components of medium frequency, the system having,

in combination, an amplifier for transmitting two of the said types, an amplifier for transmitting the third type, one of the said amplifiers transmitting the medium-frequency components, the other amplifier comprising an electron-discharge device transmitting said other-type components and having at least one control electrode controllable by a variable control bias and transmit- V ting said other-type components under control of said control bias, and means for rendering the other amplifier ineffective when the level of the medium-frequency components falls below a predetermined value said means comprising rectifying means rectifying said medium frequency components, thereby providing a control bias and means for applying said control bias to said control electrode.

4. An electric system having, in combination, means for transmitting signals of relatively-medium-frequency components and means for transmitting signals of relatively-high-frequency components, said transmission of said relativelyhigh-frequency components being accomplished by means of an electron-discharge device having at least one control electrode, the transmission of said electron-discharge device being variable in response to a variable control bias applied to said control electrode, and means effective only when the level of the relatively-medium frequency components exceed a predetermined value to control said control bias thereby controlling said electron-discharge device thereby to increase the transmission of the relatively-high-frequency components.

5. An electric system having, in combination, means for transmitting signals over a range of frequencies having a relatively-low limit and a relatively high limit, and parallel connected means controlled in accordance with the level of the signals within said frequency limits for transmitting additional signals at frequencies beyond the limits each of said transmitting means comprising a transmission channel having an input circuit and an output circuit, the input circuits of all of said transmitting means being connected in parallel and the output circuits of said transmitting means being connected in parallel.

' 6.- An electric system having, in combination, means for transmitting :unm'odulated signals over a range of frequencies having a, relatively-low limit and a relatively-high limit, parallel-connected'normally inefiective means for transmitting signals at frequencies beyond one of the limits said normally ineffective means comprising an electron-discharge device having at least one control electrode controllable by a control I bias and means controlled in accordance with the signals within said limits for varying said control bias-thereby rendering the normally ineffective means effective each of said transmitting means comprising a transmission channel having an input circuit and an output circuit, the input circuits of all of said transmitting means being connected in parallel and the output circuits of said transmitting means being connected in parallel.

7. An electric system having, Iin-eombination,

8. A system as defined in claim 7 in which said electron discharge device is of the variable-gain type.

HERMON H. SCOTT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,979,035 Hammond Oct; 30, 1934 2,003,702 Hammond July 23, 1935 2,008,708 Hammond July 23,1935 2,009,229 Hammond July 23; 1935 2,040,954 Roberts May 19, 1936 2,054,647 Ballaritine; Sept. 15, 1936 2,072,708 Case Mar. 2, 1937 2,241,581 Bouoke May 13;. 1941 2,243,921 Rust et a1 June 3, 1941 2,379,720

Koch July 3, 1945 

